It is well known that when coal with a sulphur content lower than 1% is burnt in an ordinary furnace, the amount of SO.sub.3 which is naturally generated often is not enough to reduce the ashes resistivity to such an extent as to allow an electrostatic precipitator to operate.
Said resistivity extent is about 5/10.sup.10 ohm. cm. In such cases the ashes resistivity can be increased to an acceptable degree by adding a suitable additive, such as SO.sub.3, apt to be combined with steam to generate H.sub.2 SO.sub.4.
It must be pointed out that the amount of SO.sub.3 required is relatively small, and is about 15 ppm with respect to the total weight of the smokes concerned.
Many serious problems always arise when SO.sub.3 is employed, as it is very difficult a substance to be handled. Therefore, when a huge amount of SO.sub.3 is to be used, it is usually preferable to generate it locally by melted sulphur combustion in the air, and afterwards by catalytically converting the SO.sub.2 generated into SO.sub.3. In such cases, in order to achieve a high ratio in the conversion of sulphur dioxide into sulphuric anhydride, the conversion must take place in a number of stages. Therefore the known plants apt to generate sulphuric anhydride by melted sulphur combustion prove unsuitable to be used, unchanged, to generate a small amount of gaseous sulphuric anhydride as required in combustion plants of the kind as above mentioned, particularly when coal is used as a fuel.